Transmit EQ - Some Comments by K4TAX]]

From the Omni VII Yahoo! Group - July 16, 2008

The TX EQ is a very broad range equalization system. Best described as a one that graphically appears like a shelf, hence shelf equalization. It will boost the lower frequencies, generally all below 1 KHz or will attenuate the lower frequencies, again all below 1 KHz. The end effect is an increase in low end or a decrease in low end which then gives the appearance to increased high end.

When used with very narrow band signals like PSK-31 or even MCW or RTTY, the effect of EQ is minimal to none. Although it may change the amplitude of the signal, it will not provide any realization of improvement to any degree of the signal. To the contrary, it could introduce some phase shift between two signals such as those of RTTY and actually make the signal more difficult to demodulate.

Better choices of improving ones signal is the adjustment of the TX bandwidth and the TX Low cut-off frequencies. Removing some of the higher frequencies by reducing the TX Bandwidth to say 2500 Hz and rolling off the low frequencies at 140 Hz nets a transmitted bandwidth of 2360 Hz. The male spoken voice fits well within this frequency spectrum. At the same time the effective transmitter power is concentrated over a bandwidth of only 2360 Hz. Now take another case where the transmit bandwidth is opened to 3200 Hz and the low frequency roll off is at 70 Hz, one may think this is better, however the transmit band width is now 3130 Hz. The same effective transmitter power is now distributed over a wider bandwidth. Should the receiver on the other end have an effective bandwidth of 2.1 KHz, typical of many receivers, then there is a good bit of the transmitter power, up to 1/3, that is not recovered by the receiver.

The fundamental frequencies produced by the male speaking voice cover the general range of 70 Hz to about 750 Hz. Allowing for good articulation this then requires a frequency span of 70 Hz to 2250 Hz or an effective bandwidth of 2180 Hz. Above 2250 Hz there is little energy within the spoken male voice spectrum. Below 100 Hz there is a host other noise such as breath noise, nasal noise and room noise. Those various noise components that exist are lip smacks, tongue noise, saliva clicks, bronchial roar or breath noise and other associated throat noises. Also included are noises associate with dental appliances.

Over the years, professionally, I find that EQ is one of the least understood tool by the lay person and likely one of the worst over used and mis-applied tools. At the same time, I muse at the host of extensive EQ configurations provided by some radio companies. I guess hams just like gadgets and knobs to twiddle and those do sell radios.

Bob, K4TAX

July 16, 2008

If you wish to create a "pile up bustin' signal" just set the TX Roll-off to 200 Hz and the TX Bandwidth to about 1800 Hz. The transmitted bandwidth is then 1600 Hz, to which all of the transmitter power is now concentrated. Many stations in DX competitions, due to QRM and etc, are running their receiver bandwidth in the order of 1600 to 1800 Hz. Nothing like cramming all of your transmitter power into their receiver bandwidth to be effective.

Bob, K4TAX

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